NEW YORK (CNN/Money) -
Less than a month after Hurricane Charley became one of the most expensive storms in the nation's history, Hurricane Frances could be ready to become the most costly, according to one forecast.

Michael Lewis, insurance analyst for investment bank UBS, warned in a note Wednesday that Frances, which is expected to reach the United States coast sometime Saturday, "has the potential to exceed the insured losses of Hurricane Andrew."

Andrew, the August 1992 hurricane that was the most expensive storm in history, caused about $20 billion in insured damages, adjusted for inflation.

While all the costs of Charley, which ravaged the Gulf Coast of Florida on Aug. 13, have yet to be assessed, industry damage estimates range from $6 billion up to $14 billion. Insured losses of $6 billion would put it a close third behind 1989's Hurricane Hugo, which had inflation-adjusted losses of $6.2 billion.

Despite his warning, Lewis cautioned in his note it was too soon to assess the damages that Frances could cause.

He said Allstate, Chubb Corp., St Paul Travelers and Hartford are the publicly traded insurers with greatest exposure to losses.

Because the storm is still so far off the Florida coast, Frances could veer as far south as the Florida Straits or as far north as the Carolinas, according to current projections. Voluntary evacuations were due to begin along the Florida coast Wednesday morning. (Click here for CNN.com's coverage of Hurricane Frances.)

Two factors could cause significant losses from Frances -- its strength and its current projected path, which could bring it ashore at or near the densely populated Miami-Fort Lauderdale-West Palm Beach area Saturday morning.

The National Hurricane Center estimates Frances is now a Category 4 storm, the second highest ranking, with sustained winds near 140 mph. It warned the storm could intensify before making land, though. The storm is now east of Cuba, heading towards the Bahamas.

Insurance rating agency A.M. Best Co. placed the financial strength rating of A- of Allstate's Florida unit under review with negative implications Tuesday afternoon due to the costs of Hurricane Charley, as well as the potential damage of Hurricane Frances.

Allstate (ALL: Research, Estimates), the No. 2 private insurer in the state after mutually held State Farm, reported late Tuesday that after-tax catastrophe losses related to Hurricane Charley would come to $276 million, or 40 cents a share.